This invention relates to a fuel injector, and in particular to a fuel injector suitable for use in delivering fuel under pressure to a cylinder of a compression ignition internal combustion engine.
A typical fuel injector includes a valve needle slidable within a bore and engageable with a seating to control the supply of fuel to a chamber downstream of the seating, the chamber communicating with a plurality of outlet openings. The outlet openings form a restriction to the flow of fuel and, for a given fuel pressure, serve to restrict the rate at which fuel is delivered by the injector. As the outlet openings are pre-drilled, no adjustment of the restriction to fuel flow can be made after manufacture, and the fuel flow rate cannot be adjusted in use.
In some known arrangements, for example those in which a secondary valve needle is slidable within a bore formed in the valve needle, the number of outlet openings used at any instant can be controlled. Controlling the fuel flow rate in this manner is disadvantageous, however, in that the overall spray formation or pattern changes as the number of openings in use changes, and those openings which are not in use may become blocked by coke or lacquer.
It is an object of the invention to provide an injector in which the restriction to fuel flow formed by the outlet openings of an injector, and hence the fuel flow rate, can be altered, in use, whilst using all of the outlet openings.
According to the present invention there is provided a fuel injector comprising a valve needle engageable with a seating to control the supply of fuel to an outlet opening, and an adjustment member including an opening, the adjustment member being moveable to move the opening thereof relative to the outlet opening to vary the rate at which fuel is delivered by the injector.
The adjustment member is conveniently provided with a first opening and a second opening, the adjustment member being moveable between a first position in which the first opening communicates with the outlet opening, the first opening and the outlet opening together defining a first restriction to fuel flow, and a second position in which the second opening communicates with the outlet opening, the second opening and the outlet opening together defining a second restriction to fuel flow.
The first and second openings may be of substantially equal diameter, the entry ends of the first and second openings being of different shapes to modify the restriction to fuel flow. For example, the entry end of the first opening may be sharp, the entry end of the second opening being radiused. As a result, a variation in flow rate of up to approximately 30% can be achieved.
The adjustment member may be angularly moveable or may be axially moveable between its first and second positions.
The seating may be defined by part of the adjustment member.
As the arrangements described hereinbefore permit the effective restriction to fuel flow to be adjusted, it will be appreciated that, for a given fuel pressure, the rate at which fuel is delivered by the injector can be adjusted.
In an alternative arrangement, the adjustment member may be moveable relative to the outlet opening to vary the separation of the opening of the adjustment member from the outlet opening whilst maintaining communication therebetween. Such an arrangement has the advantage that the effective restriction to flow, and hence the fuel flow rate, is continuously variable rather than being adjustable between two or more discrete levels.